The second warning raised questions about whether the FBI and CIA failed to share information.

Republican senator and member of the Senate Intelligence Committee Dan Coats said: "That's something that we have to look at.

"That's one of the key things that we have learned and need to work on to make sure it doesn't happen again, and that is simultaneous communication to all the relevant agencies when a warning is posted."

In a briefing to a House of Representatives panel, US investigators said the two suspects apparently became radicalised by information on the internet.

Officials in the United States say the surviving suspect in the Boston bombings has acknowledged his role in the attacks. However it is unclear whether the admission could be used at a criminal trial because it happened before authorities advised Dzhokhar Tsarnaev of his constitutional rights.

19-year-old bomb suspect Dzhokhar Tsarnaev Credit: Press Association

It is also unclear whether prosecutors would need the admission by Dzhokhar Tsarnaev to secure a conviction because physical evidence has already been uncovered in the investigation.

US investigators have revealed the two bombs that went off at the Boston Marathon were detonated with the kind of remote control device used for a toy car.

A trainer is seen at a makeshift memorial in Boston Credit: Reuters

US Representative Dutch Ruppersberger, the top Democrat on the House Intelligence Committee said: "It was a remote control for toy cars. Which says to me, and brother number two has said, they got the information on how to build the bomb from Inspire magazine."

Inspire was created by the American-Yemeni preacher Anwar al-Awlaki, a leader of al Qaeda's affiliate in Yemen who was killed in a U.S. drone strike. Ruppersberger said the article on bomb-building in Inspire was headlined: "How to build a bomb in your mom's kitchen."